Broadly, the invention relates to an improved apparatus for continuously mixing solid particles with a fluid composition. In a specific application, this apparatus is employed as a blender, in which sand or sand-like particles are mixed (or blended) with a gel composition, and the resulting slurry is pressurized by the mixer itself. A typical use for the resulting slurry is as a treating fluid, which is introduced into a well to enhance recovery of a petroleum product.
The sand blender described in U.S. Pat. No. 3,326,536 (Zingg et al.) is typical of conventional "closed-system" blenders now being used in oil or gas recovery operations. This patent describes two different embodiments of the blender, as illustrated in FIGS. 1 and 4. Both of these designs have certain drawbacks, which point up the need for improvement in blenders and other equipment used in downhole recovery operations.
In a continuous blending operation the objective is to be able to mix a slurry of particulate material (such as sand or other propping agents) and fluid (the gel composition, or other fluid, such as water) and pressurizing the resulting slurry to a desired level. The Zingg et al. machines have certain drawbacks which make it difficult to blend successfully the particulates and the fluid composition. For example, the structure of the mixing impeller creates a significant restriction in the flow of solids into the casing. The large surface area of the impeller and the fact that the impeller has a very close fit within the casing also contributes to frictional drag and erosion of the impeller and other parts of the blender. Also, the fluid composition tends to be trapped inside the rotating impeller, so that the recirculation required for thorough mixing of the solids and fluids is inadequate for many mixtures used in modern well treatments. As the solids to fluid ratio increases, the performance declines rapidly.
All of the embodiments described in the Zingg et al. patent require at least one fluid seal to prevent recirculation loss within the casing, as well as abrasive erosion of the blender parts. The elements of this seal must be held concentric by the impeller shaft, for proper operation and an economical operating life. This requirement dictates precisely constructed apparatus which is difficult to maintain under the harsh conditions of normal operation. As an overall problem, the blender described above can pack-off or jam with solids, which causes overheating and usually requires complete shutdown of the mixing operation.
The blender apparatus of this invention, utilizes a slinger element having a toroidal configuration, which differs in several respects from the blenders described in the Zingg et al. patent. The shape of the slinger element enables it to control the flow of fluid independently of its fit in the case. The blades of the slinger element are preferably laid out on a log spiral curve, so that they remain clear of particles, to prevent the machine from clogging. In operation, the slinger maintains a finite "eye" or opening at ambient pressure, through which solids are introduced.
The present blender also provides a positive gap or space between the blades of the slinger member and the casing. This allows the fluid to interface with the atmosphere at this gap, rather than inside the impeller, as in the Zingg et al. blenders. The slinger provides a deliberate recirculation of fluid through this gap, and the result is excellent control of the pressure distribution in the casing, as well as thorough mixing of the slurry. In contrast, prior blenders required mechanical seals to prevent recirculation, which reduces their performance and causes unacceptable wear. The combination of a vortex-type impeller and a toroidal slinger allows the blender to exhaust air through the eye. In contrast to earlier blenders, the unit will self-prime from a positive head and will not gas-lock. In the normal use of the blender, this feature is an important advantage. The blender can also mix particulates less dense than the fluid component of the mixture and discharge a well-blended slurry.